Driveshaft material is just as important as its length and diameter. Original equipment manufacturing (OEM) steel driveshafts are for just that, OEM power. An OEM shaft is rated for no more than 350 ft-lbs, or 350 to 400 hp. For high-performance use, drawn over mandrel (DOM) seamless tubing and chrome-moly steel are the two materials used. DOM steel is better than OEM steel, handling much more torque, up to 1,300 ft-lbs and 1,000 to 1,300 hp. DOM steel can be spun faster, as well, with its higher RPM rating, making it suitable for any stock LS application. This is a good choice for any car that does not need a lightweight unit.
The step up from a steel shaft is chrome-moly, which is the strongest material available. It’s used in 3,000-hp Pro Stock cars. Chrome-moly steel tubing can be heat treated as well, raising the torsional strength 22 percent and increasing the critical speed 19 percent. Steel is heavy, which increases the load on the engine bandit so that it takes the engine longer to get to speed.
Reducing driveline weight is important, so lighter materials are sometimes a better choice. Aluminum is the most common performance driveshaft material. A lightweight aluminum shaft reduces rotational mass by freeing up horsepower from the engine and reducing parasitic loss. Aluminum driveshafts are strong but cannot hold as much torque as steel. Therefore, some custom driveshaft shops do not have “twist” guarantees on aluminum driveshafts. An aluminum driveshaft supports up to 900 ft-lbs, or 900 to 1,000 hp, making it a great lightweight choice for most muscle cars.
Carbon fiber, also an option, is the most efficient in terms of parasitic loss, but it is also the most expensive; it is not needed for high-performance street use, but often is used for high power figures, up to 1,200 ft-lbs, or 900 to 1,500 hp. Carbon-fiber driveshafts are strong and have a surprisingly high torsional strength, resisting twisting and reducing the shock factor on the rear end. Carbon fiber also has the highest critical speed module of elasticity, meaning the shaft doesn’t flex at slower speeds, unlike other material components. Coupled with the highest critical speed factors and the light weight, a carbon-fiber driveshaft can free up as much as 5 hp over a stock steel driveshaft. When winning is everything, 5 hp might make the difference.
Once the driveshaft is measured and ready to build, there are a few other issues to consider. Phasing the U-joints with the weld-in yokes is an important part of the equation. With every rotation of a U-joint at any degree other than zero, a fourth-order vibration is generated. This shows up as a torsional pulse, which is felt as a significant vibration. By phasing the weld-in yokes to minimize the combined degrees of rotation, the fourth-order vibration is drastically reduced. The weld-in yokes need to be installed on the same plane; they can’t be rotated off axis of one another.
The quality of U-joints makes a difference, and not just the brand, so you need to consider the design of the U-joint as well as the load capacity. The typical choice for most cars is 1310-series U-joints; for performance applications, however, the rugged 1350-series joints are the better choice. The larger the series number, the larger the trunnion.
Trunnions are the protruding shafts that the caps ride over. Larger trunnions equate to more torsional strength. Torsional forces are exerted in a twisting motion. Changing to a larger series U-joint is not a simple task; you can’t just buy bigger joints. All yokes (slip, bolt-on, and weld-in) must match the desired joint size. You can opt for crossover U-joints, but they tend to not be as strong and they don’t last as long. This allows you to mate a larger (or smaller) U-joint to the yoke.
For example, you buy a new driveshaft that comes with 1350 weld-in yokes, but your car has 1310 yokes for the transmission and rear differential. A 1350-to-1310 joint has a 1350 on one side and a 1310 on the other, allowing you to install the driveshaft until you replace the slip and bolt-on yokes. Although it can be done, using crossover U-joints is not suggested as a long-term solution. The smaller size basically becomes a fuse and breaks eventually.
The type of joint, solid-body versus greaseable, is important as well. The Spicer-style solid-body U-joints come “lubed for life,” and do not have grease zerk fittings. This makes them a little stronger because they do not have the stress risers created by the opening for the zerk fitting in a greaseable U-joint.
Building the right driveshaft for the application is critical; every high-performance vehicle should have a driveshaft professionally built by a shop that specializes in high-performance drivelines. Have your facts straight if you are going to have a local shop build your driveshaft. The shop or builder needs to stand behind its driveshaft 100 percent. Tell the shop it is for a high-performance application, which is very different from a stock driveshaft and needs to be held to a higher standard.
Ordering a driveshaft over the Internet from a reputable high-performance builder requires accurate measurements and clear instructions of what you have and what you need. In the end, you will receive a driveline that will be perfect for your swap.